1. Field of the Disclosure
This invention is related to image sensors. In particular, embodiments of the present invention are related to stacked image sensors.
2. Background
An image capture device includes an image sensor and an imaging lens. The imaging lens focuses light onto the image sensor to form an image, and the image sensor converts the light into electric signals. The electric signals are output from the image capture device to other components of a host electronic system. The electronic system may be, for example a mobile device, a computer, a digital camera, a medical device, etc.
There is a continuing demand to decrease the size and improve the performance of image sensors. Factors that affect performance in an image sensor pixel cell include image charge transfer efficiency, image lag, and readout pixel noise. Known methods to reduce image lag include boosting the floating diffusion voltage. However, one tradeoff in boosting the floating diffusion voltage is that an additional boost line to the pixel cell may be required, which consequently requires an extra metal line for the pixel cell and therefore increases the size of the pixel cell. Another known method to reduce image lag is to increase the voltage swing of the transfer transistor of the pixel cell so that the floating diffusion voltage is boosted. However, a tradeoff with this method is that the increase of the voltage swing of the transfer transistor requires additional circuitry for the pixel cell, which increases pixel size, cost and complexity.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.